The origin of the name is uncertain. It may be from the Sanskrit "upala", meaning "stone" or "precious stone" or from opalus, the ancient Latin name for the gem (Pliny the elder, 75-79). Pliny may have also referred to the gem as paederos, but a modern commentary by Kostov (2008) questions if that name was actually applied to the opal of modern sense.

Although it is still (2007) regarded as a valid mineral species for historical reasons, Opal is not a true mineral in the accepted sense of the word as it is either composed of Cristobalite and/or Tridymite or composed of amorphous silica.

Optical Data of Opal

Type:

Isotropic

RI values:

nα = 1.400 - 1.460

Birefringence:

Opal-AG and Opal-AG are optically isotropic, but may show anomalous birefringence due to strain.
The microcrystalline varieties show birefringence: opal-C is length-fast, opal-CT is length-slow, but almost isotropic.

Kostov, Rusian I. (2008) Orphic Lithica As A Source Of Late Antiquity Mineralogical Knowledge, Annual Of The University Of Mining And Geology “ST. Ivan Rilski”, Vol. 51, Part I, Geology And Geophysics, p. 109-115.

Localities for Opal

map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality.
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(TL) indicates type locality for a valid mineral species. (FRL) indicates first recorded locality for everything else. ? indicates mineral may be doubtful at this locality. All other localities listed without reference should be considered as uncertain and unproven until references can be found.